1. Field of the Invention
The present invention is directed to a high efficiency diffractive having reduced reflection losses.
2. Description of Related Art
The use of a diffractive to split off a portion of an input beam for monitoring of power, wavelength or for other purposes is known. Often the percentage of light to be split off is very small, e.g., a couple of percent of the input light. This is due to the fact that typically most of the light is to proceed on to the actual application, and as much power as possible is to be preserved in the application beam. For such low percentage splitters, the period of the diffractive structure usually needs to be very small to eliminate excess loss to other orders. In other words, the structure needs to be small enough such that all orders above the ±1 orders are excluded in both the reflective and transmissive mode. When a diffractive having such a very small period is coated with an anti-reflective (AR) coating to reduce reflection losses, the performance of the diffractive is often degraded. This degradation is typically due to the fact that the AR coating coats the walls of the diffractive as well as the planar surfaces thereof. For a small period structure, the AR coating is thick enough relative to the period of the structure that the AR coating degrades its performance.